“I like to do law enforcement stuff,” said Mr. Sarkisian, 23, an immigrant from the republic of Georgia who is studying criminology at the city college here. “I like helping out putting bad guys in jail.”

But Mr. Sarkisian is not a police officer, and he does not carry a gun or a Taser. He is a police volunteer, part of an experiment by departments across the country that enlists trained amateurs to perform a broad — and occasionally dangerous — array of investigative duties like collecting evidence, interviewing witnesses, searching for missing persons and looking into long-dormant cases.

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But the use of volunteers in investigations raises legal and liability questions, said Robert Weisberg, the co-director of the Stanford Criminal Justice Center. He suggested such programs could provide openings for defense attorneys to suppress evidence and attack witnesses’ testimony.

“If I were a defense lawyer, I would certainly say in front of the jury, ‘Mr.’ — and I would underline Mister — ‘Mr. Shoontz, you’re a volunteer. You’re not really a police officer, are you?” Mr. Weisberg said.